An Me261 tanker maybe? But if you did that you'd probably use one version for MPA an the other as a dedicated buddy tanker.
But to be something effective and long-ranged with 30-40s tech and available in time, you need to start at about the same time as the XB-15, with the intention of eventually building something capable of carting 4+ Fritz-X or Hs 293 internally (to reduce drag) on a patrol at a minimum, and something as big as the B-36 might not be out of the question actually, if you want plenty of fuel and a good amount of bombs/missiles/depth charges. And hey, at the B-36's specs, Airborne refueling just became really practical, since you can cart 30,000+kg of fuel over the MPA version in a tanker (even if it could take anywhere from 30 to 90 minutes to transfer over that much fuel at the pressures a hose and drogue unit can take safely.)
But something a little more capable (say 6,000kg payload, 10,000km range,) than the XB-15 is entirely doable to have in service by 1940, especially if you've got more-efficient engines available, and streamline the nose of the thing, and use some tech IOTL developed for the XF-12, and comparable streamlining to further improve those metrics (and could thus legitimately hide it's design as being a pure research project and later a High-speed long-range Airliner.)